This invention relates to the synthesis of crystalline microporous solids in a growth medium. Crystalline microporous solids include, for example, silica molecular sieves and aluminosilicate zeolites.
Crystalline microporous solids are well-known for their utility as heterogeneous catalysts in industrial organic processes, such as, the alkylation of aromatic compounds with olefins; the transalkylation of aromatic compounds; the isomerization of aromatic compounds, paraffins, and olefins; the disproportionation of aromatic compounds; the cracking and hydrocracking of hydrocarbons; and the oligomerization of olefins. Moreover, zeolites and molecular sieves are useful as adsorbents for purifying gases, useful for separating mixtures of chemicals and isomers, useful as supports for catalytic metals and metal compounds, and useful for ion exchange.
Crystalline microporous solids are commonly prepared by hydrothermal methods. A typical preparation involves heating one or more nutrients, such as a source of silica and optionally a source of alumina, in water in the presence of a mineralizer and a structure directing agent until crystallization occurs. The mineralizer, which is usually hydroxide, functions as a solubilizer of silica and alumina transporting them through the reactive solution or gel to nucleation sites. The structure directing agent includes templates and stabilizers. The template, which may be a cation or neutral species, tends to favor the nucleation and growth of a particular zeolite. The stabilizer, often referred to as a pore filler, functions in a stabilizing role and may be required for a successful synthesis. Water and organic bases, such as primary, secondary and tertiary aliphatic amines and tetraalkylammonium halides, are common stabilizers. Additionally, the template and stabilizer may assist in controlling pH and/or may provide charge balance with a counteranion or anionic framework. Several typical hydrothermal methods of preparing crystalline microporous solids are described hereinbelow.
U.S. Pat. Nos. 4,942,027 and 4,795,623 disclose a hydrothermal synthesis of sodium ferrierite. The synthesis involves adding sources of sodium, aluminum and silicon to an excess of water in the presence of a nitrogen-containing compound, such as pyridine, to form a mixture which is thereafter heated for a period of time under autogenous pressure to effect crystallization.
U.S. Pat. No. 4,578,259 teaches the synthesis of a siliceous crystalline aluminosilicate designated "ISI-6." The synthesis comprises forming an aqueous mixture containing sources of silica and alumina, a source of alkali metal, pyridine, an oxygen-containing organic component, such as an alcohol, and a nitrogen-containing component other than pyridine, such as isopropylamine, and subjecting the mixture to a temperature ranging from 100.degree. C. to 300.degree. C. until the crystalline aluminosilicate is formed.
W. J. Smith et al. teach in the Journal of the Chemical Society, Faraday Transactions I, 85, (1989) 3623, that zeolites ferrierite, mordenite, and ZSM-5 are crystallized from an aqueous growth medium comprising silica, alumina, sodium oxide, and pyridine. The ferrierite is limited to silica to alumina molar ratios greater than 28.6. Crystal sizes range from 5 microns (.mu.m) to 20 .mu.m.
Among the known hydrothermal methods are some which employ fluoride as a mineralizer. European patent application No. 337,479A, for example, discloses the use of hydrogen fluoride or fluorine-containing compounds in water at low pH to mineralize the silica in glass for a synthesis of zeolite ZSM-5.
Z. Daqing et al. disclose in the Journal of the Chemical Society, Chem. Communications, 1990, 884, the preparation of an aluminosilicate zeolite designated "CJS-1" from an aqueous medium containing hydrogen fluoride and piperazine. The molar composition of the reaction mixture comprises 0.5 piperazine, 0.04 alumina (Al.sub.2 O.sub.3), 1.0 silica (SiO.sub.2), 1.0 hydrogen fluoride, and 30 water. Thus, the water to silica molar ratio, H.sub.2 O/SiO.sub.2, is 30/1 and the water to piperazine molar ratio is 60/1.
French Patents 2,632,943 and 2,631,621 disclose the synthesis of zeolites having structures designated MTT and TON under hydrothermal conditions in the presence of fluoride as a mineralizer. The synthesis mixture contains water, a source of silica, possibly a source of an aluminum salt, a source of a mobilizing agent containing fluoride ion, such as hydrogen fluoride, and a source of a structuring agent capable of supplying organic cations, such as isopropylamine. It is taught that the water to silica molar ratio ranges from 6 to 200, preferably from 15 to 80, and the organic structuring agent to silica molar ratio ranges from 0.1 to 6, preferably from 1 to 5. Thus, the water to organic structuring agent molar ratio may range from 1 to 2000, preferably from 3 to 80. It is further disclosed that fluoride may replace oxide in the framework of the crystalline zeolite formed. The crystal size of these zeolites is taught to range from 0.1 .mu.m to 250 .mu.m, preferably from 2 .mu.m to 130 .mu.m.
There are disadvantages to synthesizing microporous solids by hydrothermal methods. Typically, the crystalline products of such systems are formed under highly metastable conditions defined by a large number of variables. As a result, the crystallization of mixed phases is not uncommon and the purity of the crystalline solid can be compromised. In addition, in aqueous media employing hydroxide as the mineralizer the pH is limited to the basic range, thus restricting the silica and alumina precursor species which form the crystalline products.
As an even more significant disadvantage, nucleation and growth of crystalline microporous solids in aqueous media are uncontrolled resulting in random and unconstrained formation of small crystals and microcrystalline aggregates. Thus, hydrothermal methods usually yield crystals ranging in size from about 0.1 .mu.m to about 60 .mu.m and rarely exceeding 100 .mu.m. In applications in the fields of catalysis, adsorption, separations, and catalytic supports, it is beneficial for the microporous solid to have a high surface area and a microcrystalline nature. Disadvantageously, however, certain high technology solid-state applications cannot be achieved with small crystals in the micron-size range.
High technology, solid-state applications for microporous solids can be found in the fields of membrane technologies, molecular electronics, non-linear optical materials, chemical sensors and advanced batteries, as disclosed by G. A. Ozin, A. Kuperman and A. Stein in the review article "Advanced Zeolite Materials Science," Angewante Chemie International Edition English, 28 (1989), 359-376. Such applications will rely on introducing insulator, semiconductor or metallic guest materials either into the framework or into the extra framework sites of a microporous solid which acts as a host. Alternatively, thin films of such guest-host composites may be required. It is generally believed that large single crystals on the order of at least 100 .mu.m, that is 0.1 millimeter (mm), and preferably at least 0.3 mm, are needed if the electronic, optical, magnetic, and chemical properties of crystalline microporous host solids and their guest--host composites are to be successfully exploited for these advanced technologies.
In recent years the synthesis of crystalline microporous solids has been investigated in aqueous-organic solvent mixtures as well as in non-aqueous media. The number of precursor species may be reduced in a non-aqueous system, thereby producing purer crystalline phases. See, for example, G. Boxhoorn et al. in the Journal of the Chemical Society, Chemical Communications, 1983, 1416, who studied the influence of organic additives on the precursor species in an aqueous ZSM-5 synthesis. In addition, silica and alumina are expected to exhibit different solubilities in non-aqueous solvents, while the precursor species are expected to exhibit different diffusion rates. Changes in the solubilities and diffusion rates are expected to affect nucleation and crystallization processes, but not in a predictable manner.
D. M. Bibby et al. disclose in Nature, 317, (1985) 157, that sodalite can be prepared either in a silica-rich aluminosilicate form or in a pure-silica form from an "essentially non-aqueous solvent system," exemplified by ethylene glycol or propanol. Sodium hydroxide is the disclosed mineralizer. The crystal size is taught to be no greater than about 30 .mu.m.
E. Pernklau discloses in N. Jb. Miner. Mh., 9, (1989) 385, that sodalite is obtained from a variety of organic solvents, including s-butyl alcohol, glycerol, hexamethyleneimine, sulfolane, triethylenediamine, and various glycols. Sodium hydroxide is employed as a mineralizer, and the solvents are taught to be "anhydrous of analytical grade."
H. Qisheng et al. disclose in Zeolites: Facts, Figures, Future, Elsevier Science Publishers B.V., Amsterdam, 1989, pp. 291f, that pentasil zeolites, namely silicalite, ZSM-39 and ZSM-48, can be grown in organic solvents in the presence of templating agents or crystal seeds. Specific solvents include glycol, glycerol, sulfolane, dimethylsulfoxide, ethanol, pyridine, and C.sub.6-7 alcohols. The crystalline product and its ring size (4, 5, 6) depend upon the amount of alkali and specific templating agent employed. Crystals in the size range from about 20 .mu.m to about 30 .mu.m are taught.
Disadvantageously, the above-described processes employing aqueous-organic solvent mixtures or non-aqueous growth media do not necessarily lead to fewer, purer crystal phases. Moreover, heretofore the crystals obtained from aqueous-organic solvent mixtures and non-aqueous growth media are not larger in size than the crystals obtained from hydrothermal methods. Thus, prior art methods do not yield crystals which are useful for solid-state, high technology applications. Accordingly, a need exists to find a general method of growing crystalline microporous solids which leads to purer phases and crystals of at least about 0.1 mm, and preferably at least about 0.3 mm, in size.